(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of creating a reliable, fine pitch solder bump at low cost.
(2) Description of the Prior Art
Semiconductor device packaging has over the years received increased emphasis due to a continued decrease in semiconductor device feature size, a decrease that is driven by the dual requirements of improved device performance and reduced device manufacturing cost. This trend has led to a significant increase in semiconductor device density, which places increased emphasis on device or package I/O capabilities. The metal connections, which connect the Integrated Circuit to other circuits or to system components, have therefore become more important and can, with further miniaturization of the semiconductor device, have an increasingly negative impact on circuit performance. Increasing parasitic capacitance and resistance of the metal interconnections can significantly degrade chip performance. Of most concern in this respect is the voltage drop along the power and ground buses and the RC delay of the critical signal paths. Attempts to reduce the resistance by using wider metal lines result in higher capacitance of these wires.
One of the approaches that has been taken to solve these packaging problems is to develop low resistance metals (such as copper) for the interconnect wires, while low dielectric constant materials are being used in between signal lines. Another approach to solve problems of I/O capability has been to design chips and chip packaging techniques that offer dependable methods of increased interconnecting of chips at a reasonable manufacturing cost. This has led to the development of Flip Chip Packages.
Flip-chip technology fabricates bumps (typically Pb/Sn solders) on Al pads on the chips and interconnects the bumps directly to the package media, which are usually ceramic or plastic based. The flip-chip is bonded face down to the package medium through the shortest paths. These technologies can be applied not only to single-chip packaging, but also to higher or integrated levels of packaging in which the packages are larger, and to more sophisticated substrates that accommodate several chips to form larger functional units.
The flip-chip technique, using an array of I/O interconnects has the advantage of achieving the highest density of interconnection to the device combined with a very low inductance interconnection to the package. However, pre-testability, post-bonding visual inspection, and Coefficient of Thermal Expansion (CTE) matching to avoid solder bump fatigue are still challenges. In mounting several packages together, such as surface mounting a ceramic package to a plastic board, the TCE mismatch can cause a large thermal stress on the solder-lead joints that can lead to joint breakage caused by solder fatigue from temperature cycling operations.
Prior Art substrate packaging uses ceramic and plastic flip chip packaging. Ceramic substrate packaging is expensive and has proven to limit the performance of the overall package. Recent years has seen the emergence of plastic substrate flip chip packaging, this type of packaging has become the main stream design and is frequently used in high volume flip chip package fabrication. The plastic substrate flip chip package performs satisfactorily when used for low-density flip chip Integrated Circuits (IC's). If the number of pins emanating from the IC is high, that is in excess of 350 pins, or if the number of pins coming from the IC is less than 350 but the required overall package size is small, the plastic flip chip structure becomes complicated and expensive. This can be traced to the multi-layer structure used to create the plastic flip chip package. This multi-layer structure results in a line density within the package of typically 2-3 mil range. This line density is not sufficiently high for realizing the fan out from the chip I/O to the solder balls on the package within a single layer, leading to the multi-layer approach. The multi-layer approach brings with it the use of relatively thick (50 to 75 μm) dielectric layers, these layers have a Coefficient of Thermal Expansion (CTE) that is considerably higher than the CTE of the laminate board on which the plastic flip chip package is mounted. To counteract this difference in CTE's the overall package must be (thermally and mechanically) balanced resulting in the use of additional material and processing steps to apply these materials, increasing the cost of the Ball Grid Array (BGA) package and creating a yield detractor.
In creating semiconductor devices, the technology of interconnecting devices and device features is a continuing challenge in the era of sub-micron devices. Bond pads and solder bumps are frequently used for this purpose, whereby continuous effort is dedicated to creating bond pads and solder bumps that are simple, reliable and inexpensive.
Bond pads are generally used to wire device elements and to provide exposed contact regions of the die. These contact regions are suitable for wiring the die to components that are external to the die. An example is where a bond wire is attached to a bond pad of a semiconductor die at one end and to a portion of a Printed Circuit Board at the other end of the wire. The art is constantly striving to achieve improvements in the creation of bond pads that simplify the manufacturing process while enhancing bond pad reliability.
Materials that are typically used for bond pads include metallic materials, such as tungsten and aluminum, while heavily doped polysilicon can also be used for contacting material. The bond pad is formed on the top surface of the semiconductor device whereby the electrically conducting material is frequently embedded in an insulating layer of dielectric. In using polysilicon as the bond pad material, polysilicon can be doped with an n-type dopant for contacting N-regions while it can be doped with p-type dopant for contacting P-regions. This approach of doping avoids inter-diffusion of the dopants and dopant migration. It is clear that low contact resistance for the bond pad area is required while concerns of avoidance of moisture or chemical solvent absorption, thin film adhesion characteristics, delamination and cracking play an important part in the creation of bond pads.
The conventional processing sequence that is used to create an aluminum bond pad starts with a semiconductor surface, typically the surface of a silicon single crystalline substrate. A layer of Intra Metal Dielectric (IMD) is deposited over the surface, a layer of metal, typically aluminum, is deposited over the surface of the layer of IMD. The layer of metal is patterned and etched typically using a layer of photoresist and conventional methods of photolithography and etching. After a bond pad has been created in this manner, a layer of passivation is deposited over the layer of IMD. An opening that aligns with the bond pad is created in the layer of passivation, again using methods of photolithography and etching.
A conventional method that is used to create a solder bump over a contact pad is next highlighted. FIGS. 1 through 4 show an example of one of the methods that is used to create an interconnect bump. A semiconductor surface 10 has been provided with a metal contact pad 14, the semiconductor surface 10 is protected with a layer 12 of passivation. An opening 19 has been created in the layer 12 of passivation, the surface of the metal contact pad 14 is exposed through this opening 19. Next, FIG. 2, a dielectric layer 16 is deposited over the surface of the layer 12 of passivation. The layer 16 of dielectric is patterned and etched, creating an opening 21 in the layer 16 of dielectric that aligns with the metal pad 14 and that partially exposes the surface of the metal pad 14. A layer 18 of metal, typically using Under-Bump-Metallurgy (UBM), is created over the layer 16 of dielectric, layer 18 of metal is in contact with the surface of the metal pad 14 inside opening 21. The region of layer 18 of metal that is above the metal pad 14 will, at a later point in the processing, form a pedestal over which the interconnect bump will be formed. This pedestal can be further extended in a vertical direction by the deposition and patterning of one or more additional layers that may contain a photoresist or a dielectric material, these additional layers are not shown in FIG. 2. These layers essentially have the shape of layer 16 and are removed during one of the final processing steps that is applied for the formation of the interconnect bump.
A layer of photoresist (not shown) is deposited, patterned and etched, creating an opening that aligns with the contact pad 14. A layer 20 of metal, such as copper or nickel, FIG. 3, that forms an integral part of the pedestal of the to be created interconnect bump, is next electroplated in the opening created in the layer of photoresist and on the surface of the layer 18 of metal, whereby the layer 18 serves as the lower electrode during the plating process. Layer 20 in prior art applications has a thickness of between about 1 and 10 μm with a typical value of about 5 μm. The final layer 22 of solder is electroplated on the surface of layer 20. The patterned layer of photoresist is then removed.
The layer 18 of metal is next etched, FIG. 4, leaving in place only the pedestal for the interconnect bump. During this etch process the deposited layers 20 and 22 serve as a mask. If, as indicated above, additional layers of dielectric or photoresist have been deposited for the further shaping of pedestal 18 in FIG. 2, these layers are also removed at this time.
A solder paste or flux (not shown) is now applied to the layer 22 of solder, the solder 22 is melted in a reflow surface typically under a nitrogen atmosphere, creating the spherically shaped interconnect bump 22 that is shown in FIG. 4.
In addition to the above indicated additional layers of dielectric or photoresist that can be used to further shape the pedestal of the interconnect bump, many of the applications that are aimed at creating interconnect bumps make use of layers of metal that serve as barrier layers or that have other specific purposes, such as the improvement of adhesion of the various overlying layers or the prevention of diffusion of materials between adjacent layers. These layers collectively form layer 18 of FIG. 4 and have, as is clear from the above, an effect on the shape of the completed bump and are therefore frequently referred to as Ball Limiting Metal (BLM) layer. Frequently used BLM layers are successive and overlying layers of chrome, copper and gold, whereby the chrome is used to enhance adhesion with an underlying aluminum contact pad, the copper layer serves to prevent diffusion of solder materials into underlying layers while the gold layer serves to prevent oxidation of the surface of the copper layer. The BLM layer is layer 18 of FIGS. 2 through 4.
Increased device density brings with it increased closeness of components and elements that are part of the created semiconductor devices. This increased closeness is expressed as a reduction in the spacing or “pitch” between elements of a semiconductor device. State-of-the-art technology uses solder bumps having a pitch of about 200 μm, which imposes a limitation on further increasing device density. The limitation in further reducing the pitch of solder bumps is imposed by concerns of reliability, which impose a relatively large ball size for the solder bump. This relatively large solder ball restricts further reducing the solder ball pitch.
In the majority of applications, solder bumps are used as interconnections between I/O bond pads and a substrate or printed circuit board. Large solder balls bring with it high standoff since a solder ball with high standoff has better thermal performance (CTE mismatching is easier to avoid resulting in reduced thermal stress on the solder balls). Large solder balls are therefore required in order to maintain interconnect reliability. Low-alpha solder is applied to avoid soft error (electrical or functional errors) from occurring, thereby eliminating the potential for inadvertent memory discharge and incorrect setting of the voltage (1 or 0).
U.S. Pat. No. 6,162,652 (Dass et al.) provides for the testing of an integrated circuit device including depositing a solder bump on a surface of a bond pad.
U.S. Pat. No. 5,756,370 (Farnworth et al.) provides a compliant contact system for making temporary connection with a semiconductor die for testing and a method for fabricating the pliable contact system.
U.S. Pat. No. 5,554,940 (Hubacker) addresses the probing of semiconductor devices that have been provided with contact bumps and the formation of peripheral test pads.
U.S. Pat. No. 5,665,639 (Seppala et al.), U.S. Pat. No. 6,051,450 (Ohsawa et al.) and U.S. Pat. No. 5,882,957 (Lin) show related bump processes.
U.S. Pat. No. 5,633,535 (Chao et al.) shows a pedestal process using dry resist.
U.S. Pat. No. 6,103,552 (Lin) provides a process and package for achieving wafer scale packaging, which includes formation of a solder bump.